A dry developing method utilizing an electrophotographic system serves to directly stick toner powder to an electrostatic latent image on a photosensitive acceptor, thereby forming an image. A two-component type developer and a one-component type developer comprising only toner particles are used for the developers. The two-component type developer easily maintains the toner particles in a stable charging state. For this reason, recently, a reliable two-component type developer has often been used. The two-component type developer contains toner particles and carrier particles. The carrier particles serve to give a desirable electric charge to the toner particles and to deliver the charged toner particles to a photosensitive acceptor. Moreover, the toner particles contain a colorant and can be selectively stuck to an electrostatic latent image formed on the photosensitive acceptor to form an image, and can fix the image thus formed to a recording medium such as a paper.
Conventionally, a dry developing method utilizing an electrophotographic system has mostly been utilized for monochrome copying and printing. In recent years, the dry developing method has also been utilized in color copying and printing. Furthermore, a developing apparatus has been diversified. For this reason, the design of a developer has been required that is suitable for various processes. In particular, a toner particle to be used for the color copying and printing has a smaller particle size than that of a toner particle to be used in the conventional monochrome copying and printing in order to meet a very stringent requirement for an enhancement in picture quality in a market. In order to exhibit a charging property required for the toner particle thus made fine, the surface of the toner particle is modified. There occurs a problem in that a characteristic such as a water absorptivity is remarkably changed depending on the surface modification or the like.
On the other hand, it is necessary to manufacture a toner particle having a small particle size and a sharp particle size distribution with a high yield. Therefore, a toner particle manufactured by a polymerizing method has been used more often than a toner particle manufactured by a grinding method according to the conventional art.
Referring to a polymerized toner, a resin particle obtained by emulsion polymerization, suspension polymerization or the like is used as a toner particle. In such a polymerized toner, an emulsifying agent, a dispersing agent or the like used in the manufacture of the resin particle remains on the surface of the resin particle. A characteristic such as a water absorptivity of the toner particle is changed depending on such a component stuck to the surface.
When the water absorptivity of the toner particle is thus changed, the charging amount of a developer remarkably fluctuates depending on a use environment so that a dependency on an environment is increased. On a high temperature and high humidity condition (hereinafter referred to as an “HH condition”) (for example, 35° C. and 80% RH), consequently, the charging amount of a developer is reduced so that an image defect such as a fog or toner scattering is apt to occur. On a low temperature and low humidity condition (hereinafter referred to as an “LL condition”) (for example, 10° C. and 20% RH), furthermore, the storage of the electric charge of a developer, that is, a so-called charge-up phenomenon easily occurs so that an image defect such as an insufficient image density is apt to be caused. On the other hand, Japanese Laid-Open Patent Publication No. 2000-10341 has proposed to employ a method of carrying out filter-off by filter pressing when filtering a colored resin particle from a liquid medium in respect to an uniform wash of the surface of a toner particle in order to relieve a change in a characteristic caused by a fluctuation in the use environment of the polymerized toner. In this method, however, a reduction in the charging amount on the HH condition cannot be suppressed. In recent years, furthermore, a demand for more stabilizing the characteristic of a developer against a change in an environment has been increased. Only an improvement described in the above publication has been insufficient for meeting a recent strict demand.
On the other hand, a carrier particle forming a two-component type developer is roughly divided into an iron powder carrier, a ferrite carrier, a magnetite carrier, a complex carrier and the like. Recently, a resin coated carrier obtained by utilizing the above materials as cores to coat a surface with a resin has widely been used. In such a conventional resin coated carrier, a water absorptivity is changed in the same manner as a toner. For this reason, there occurs a problem in that a charging amount is greatly varied with a change in an environment and the same image defect is caused. In the carrier, particularly, the water absorptivity on the HH condition is increased so that the electric resistance of the carrier is remarkably reduced. Consequently, an electric charge leakage phenomenon is apt to occur. In a developer containing such a carrier particle, there occurs a problem in that an image defect or carrier sticking is caused. On the other hand, Japanese Laid-Open Patent Publication No. Hei 8-62899 has proposed that two kinds of carriers having different environmental variation rates are mixed for use in order to relieve the dependency on an environment of a resin coated carrier. In the method described in the publication, however, it is hard for a developer to meet the demand of the market in a situation in which the dependency on an environment of a toner is deteriorated by a reduction in the particle size of the toner and the advent of the polymerized toner as described above.
Moreover, Japanese Laid-Open Patent Publication No. Hei 11-295934 has described that the rate of metal atoms such as iron or alkaline metal present on the surface of a carrier particle is set to be 7 to 20 number % so that it is possible to obtain an advantage that the storage of an electric charge can be prevented and a stable image can be obtained on the LL condition. However, a developer using such a carrier has a problem in that an electric charge is apt to leak and a desirable charging amount cannot be obtained on the HH condition, resulting in the generation of a fog or the like.
In order to relieve a change in a developer characteristic with a variation in an environment (a dependency on an environment), a developer in which the characteristics of a toner particle and a carrier particle are individually examined has been prepared in consideration of a system using the developer. Even if the respective characteristics are thus examined individually, the characteristic of the developer is not always improved as expected in many cases.
As a result of the investigation of this phenomenon, it has been found that the dependency on an environment of the developer is related to not only the independent characteristic of the toner particle or the carrier particle but also the relationship between the characteristic of the toner particle and that of the carrier particle.
More specifically, the developer is used on the HH condition (for example, 35° C. and 80% RH) and the LL condition (for example, 10° C. and 20% RH) as well as a normal temperature and normal humidity condition (hereinafter referred to as an “NN condition”, for example, 23° C. and 60% RH). For this reason, it is preferable that a difference in the characteristic of the developer used on the HH condition and the LL condition should be small.
Conventionally, toner particles having characteristics changed on the HH and LL conditions with difficulty have been prepared and carrier particles having characteristics changed on the HH and LL conditions with difficulty have been prepared, and some of them having excellent characteristics have been combined for use. In some cases in which a carrier particle having a very small variation range in an environment is used, however, the excellent characteristic of the carrier particle cannot be exhibited depending on the toner particle to be used. Furthermore, the characteristic of the whole developer in most cases is more deteriorated than that estimated from the toner particle or the carrier particle, respectively. Taking note of a difference in the amount of a water absorption between a toner particle and a carrier particle to improve the dependency on an environment of a developer, Japanese Laid-Open Patent Publication No. Hei 8-30022 has disclosed the invention of a “two-component type developer for electrostatic development comprising at least a toner and a carrier having |x-y|≦0.15 (μg/cm2) and x≦0.5 (μg/cm2), wherein a water absorptivity of the toner is represented by x and a water absorptivity of the carrier is represented by y”. However, “the amount of a water adsorption per unit area” described in the publication is calculated by using a false surface area obtained by the assumption that a toner particle and a carrier particle are single dispersed spheres. However, it is apparent that the toner particle and the carrier particle, which are actually used, have a distribution of particle sizes and a specific surface area is varied depending on the state of a surface. In other words, the surface area described in the publication is calculated on the assumption that equal mean particle sizes have the same value even if the distribution of the particle size and the state of the surface are different. In an actual particle, however, the effective surface area of a carrier is substantially changed greatly depending on the irregularities of the surface of a core material, resin coating, baking of resin coating and the like. Accordingly, “the amount of a water adsorption per unit, area” described in the publication is not substantial. For this reason, even if the developer is prepared according to the description of the publication, a change in the charging amount with an environmental variation cannot be reduced.
The present invention overcomes these problems in the prior art by providing a dry two-component type developer for electrophotography in which a dependency on an environment is reduced, the selecting conditions of toner particle and a carrier particle are set, and the dependency on an environment obtained in accordance with the set conditions is small and high image quality having neither a fog nor toner scattering can also be obtained at a high temperature and high humidity.
The present invention also provides a dry two-component type developer for electrophotography which can obtain high picture quality having no carrier sticking by controlling the water absorptivity of a carrier particle.
The present invention further provides a dry two-component type developer for electrophotography which can prevent a defective delivery on a high temperature and high humidity condition and a defective mixture on a low temperature and low humidity condition and can obtain an image of high quality.
The present invention still further provides a dry two-component type developer for electrophotography in which the dependency on an environment of a charging amount is reduced so that a fog and toner scattering are prevented under a high temperature and a high humidity and a sufficient image density can be secured under a low temperature and a low humidity, and furthermore, high picture quality having no carrier sticking can be obtained for a long period of time.